The present invention relates generally to the field of television receivers and, more particularly, to improved circuits used in television receivers for compensating for variations in CRT (cathode ray tube) electron gun characteristics.
It is known that the characteristics of the electron guns used in the CRT's of television receivers vary with time as the receiver is used, this variation being commonly manifested in a time varying cathode current versus cathode voltage relationship. More particularly, during the initial burn-in period of the receiver, about 150-200 hours, the cathode current versus cathode voltage relationship of the CRT electron gun is represented by a relatively stable curve such as curve 10 illustrated in FIG. 1. Curve 10 is characterized by a cut-off point 12 corresponding to a cathode voltage in response to which nearly zero cathode current is produced and a particular gain or slope, these characteristics being largely determined by the physical construction of the electron gun and cathode as well as the grid voltages of the CRT. After the initial burn-in period of the receiver, it has been found that the cathode voltage cut-off point of the electron gun may shift such as to a point 14 while the gain or slope of the cathode voltage versus cathode current curve remains substantially unmodified. Therefore, following the burn-in period, the cathode voltage versus cathode current curve may assume, for example, the position illustrated by curve 16 of FIG. 1. As the CRT ages further, the cut-off point 14 remains relatively stable but now the gain of the cathode I-V curve begins to change. Thus, with time, the cathode I-V curve may assume the forms illustrated by curves 18 and 20 of FIG. 1.
The changing cathode I-V characteristic curve of the CRT has a deleterious effect on the image reproduced by the television receiver in that the response of the electron gun cathode to any particular level of video signal is not fixed. For example, assume that a predetermined video signal level, as represented by a corresponding level of cathode voltage, results in the development of a cathode current adapted for producing a white picture signal on the television receiver screen when the CRT is characterized by curve 10 of FIG. 1. Now, as the cathode I-V curve changes in response to cathode aging, the same level of video signal will result in the development of smaller cathode current levels so that picture signals differing from the originally produced white picture signal will be displayed on the CRT screen. This is, of course, a highly undesireable effect in that it reduces the predictability of the qualities of a reproduced image in response to a given video signal.
Prior art techniques utilized to at least partially overcome the foregoing problem include circuits operable for manually adjusting the cathode gain and cut-off characteristics to compensate for aging changes. Such adjustments, however, are normally performed at the factory after the initial burn-in period of the receiver so that no compensation is effected for subsequently occurring changes. Examples of such manual adjustment circuits are disclosed in U.S. Pat. Nos. 3,598,905, 3,705,259, 3,670,095, 3,612,757 and 3,820,155.
Also, as disclosed in an article entitled "Automatic Stabilization of Background Color in Color TV Receivers" published on pages 8-12 of the February, 1977 issue of IEEE Transactions on Consumer Electronics, it is known to provide a circuit which automatically compensates for changes in the cut-off point of the cathode I-V curve. Other similar circuits are disclosed in U.S. Pat. Nos. 4,160,995, 3,976,836, 3,855,614 and 3,737,562. These circuits do not, however, provide compensation for changes in the gain of the curve. As exemplified by U.S. Pat. Nos. 4,012,775 and 4,070,691, it is also known to adjust both the gain and offset of a video signal in an attempt to automatically compensate for changes in the cut-off point and gain of the I-V curve of a CRT cathode. However, the circuits disclosed in these patents tend to be relatively unstable since the adjustments are not made in response to levels of cathode current which are fixed at desired values.
It is accordingly a primary object of the present invention to provide a novel and improved CRT tracking circuit which is highly stable in operation and adapted for automatically compensating for changes in both the cut-off point and the gain of the I-V curve of a CRT cathode, which changes may result from aging or the like.